Evaporator blower function

[Flynavy]

I have a 74 TWind with a factory Armstrong ac. So for it has been working ok considering the heat we have been having. I've been running it quite a bit while working on the inside and it cools down well enough. The drain tube is open although the inside cover seems to sweat a little when I have the door open.

I took the shroud off to clean it up a little and touch up the paint. Before putting the shroud back on I ran it and took a look around. I notice the evaporator blower doesn't come on. Not being sure of how the blower should function I don't know what to make of this.

Can someone explain to me how or when the blower should operate?

Thanks, Paul